ixbin: Add util script to update image contents

[wkz]

Description

usage: ixbin COMMAND [ARGS...]

Update contents of Infix images

Commands:

  unpack PKG KEY-DIR DIR

    Unpack the RAUC bundle PKG and the root filesystem contained in
    it, using the keys stored in KEY-DIR, and store the results in
    DIR.

  pack DIR [KEY-DIR] PKG

    Repackage the (possibly modified) root filesystem in DIR, updating
    all signatures as necessary using the keys stored in KEY-DIR (or
    the keys used to unpack the bundle, if not specified) and store
    the resulting bundle in PKG.

  exec DIR COMMAND [ARGS...]

    Run COMMAND, possibly with ARGS, from the root of the filesystem
    in DIR, inside of a fakeroot context that allows otherwise
    privileged operations such as ownership modifications.

  help

    Show this message and exit.

Key directories:

  KEY-DIR arguments must reference a directory that contains the
  public/private keypair used to verify/sign an image. Most commonly,
  board/common/signing-keys/development from the Infix source tree is
  used to validate the input image.

Example:

  Include a custom file inside a prebuilt Infix image, assign it to
  the UID/GID 1337; and package up the results, signing it with the
  same developer key that was used to create it:

    ixbin unpack infix-aarch64.pkg ~/infix/board/common/signing-keys/development tmp
    cp VBRUN300.DLL tmp/rootfs/lib/
    ixbin exec tmp chown 1337:1337 lib/VBRUN300.DLL
    ixbin pack tmp infix-vb3-aarch64.pkg

Checklist

Tick relevant boxes, this PR is-a or has-a:

• [ ] Bugfix
  • [ ] Regression tests
  • [ ] ChangeLog updates (for next release)
• [ ] Feature
  • [ ] YANG model change => revision updated?
  • [ ] Regression tests added?
  • [ ] ChangeLog updates (for next release)
  • [ ] Documentation added?
• [ ] Test changes
  • [ ] Checked in changed Readme.adoc (make test-spec)
  • [ ] Added new test to group Readme.adoc and yaml file
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (please detail in commit messages)
• [ ] Build related changes
• [ ] Documentation content changes
  • [ ] ChangeLog updated (for major changes)
• [x] Other (please describe): Utility script